Nuclear reactors are used in central-station electric power generation, research and propulsion. A reactor pressure vessel contains the reactor coolant, i.e. water, which removes heat from the nuclear core. Respective piping circuits carry the heated water or steam to the steam generators or turbines and carry circulated water or feedwater back to the vessel. Operating pressures and temperatures for the reactor pressure vessel are about 7 MPa and 288.degree. C. for a boiling water reactor (BWR), and about 15 MPa and 320.degree. C. for a pressurized water reactor (PWR). The materials used in both BWRs and PWRs must withstand various loading, environmental and radiation conditions.
Some of the materials exposed to high-temperature water include carbon steel, alloy steel, stainless steel, and nickel-based, cobalt-based and zirconium-based alloys. Despite careful selection and treatment of these materials for use in water reactors, corrosion occurs on the materials exposed to the high-temperature water. Such corrosion contributes to a variety of problems, e.g., stress corrosion cracking, crevice corrosion, erosion corrosion, sticking of pressure relief valves and buildup of the gamma radiation-emitting Co-60 isotope.
Stress corrosion cracking (SCC) is a known phenomenon occurring in reactor components, such as structural members, piping, fasteners, and welds, exposed to high-temperature water. As used herein, SCC refers to cracking propagated by static or dynamic tensile stressing in combination with corrosion at the crack tip. The reactor components are subject to a variety of stresses associated with, e.g., differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stress from welding, cold working and other asymmetric metal treatments. In addition, water chemistry, welding, heat treatment, and radiation can increase the susceptibility of metal in a component to SCC.
It is well known that SCC occurs at higher rates when oxygen is present in the reactor water in concentrations of about 5 ppb or greater. SCC is further increased in a high radiation flux where oxidizing species, such as oxygen, hydrogen peroxide, and short-lived radicals, are produced from radiolytic decomposition of the reactor water. Such oxidizing species increase the electrochemical corrosion potential (ECP) of metals. Electrochemical corrosion is caused by a flow of electrons from anodic to cathodic areas on metallic surfaces. The ECP is a measure of the thermodynamic tendency for corrosion phenomena to occur, and is a fundamental parameter in determining rates of, e.g., SCC, corrosion fatigue, corrosion film thickening, and general corrosion.
In a BWR, the radiolysis of the primary water coolant in the reactor core causes the net decomposition of a small fraction of the water to the chemical products H.sub.2, H.sub.2 O.sub.2, O.sub.2 and oxidizing and reducing radicals. For steady-state operating conditions, equilibrium concentrations of O.sub.2, H.sub.2 O.sub.2, and H.sub.2 are established in both the water which is recirculated and the steam going to the turbine. This concentration of O.sub.2, H.sub.2 O.sub.2, and H.sub.2 is oxidizing and results in conditions that can promote intergranular stress corrosion cracking (IGSCC) of susceptible materials of construction. One method employed to mitigate IGSCC of susceptible material is the application of hydrogen water chemistry (HWC), whereby the oxidizing nature of the BWR environment is modified to a more reducing condition. This effect is achieved by adding hydrogen gas to the reactor feedwater. When the hydrogen reaches the reactor vessel, it reacts with the radiolytically formed oxidizing species on metal surfaces to reform water, thereby lowering the concentration of dissolved oxidizing species in the water in the vicinity of metal surfaces. The rate of these recombination reactions is dependent on local radiation fields, water flow rates and other variables.
The injected hydrogen reduces the level of oxidizing species in the water, such as dissolved oxygen, and as a result lowers the ECP of metals in the water. However, factors such as variations in water flow rates and the time or intensity of exposure to neutron or gamma radiation result in the production of oxidizing species at different levels in different reactors. Thus, varying amounts of hydrogen have been required to reduce the level of oxidizing species sufficiently to maintain the ECP below a critical potential required for protection from IGSCC in high-temperature water. As used herein, the term "critical potential" means a corrosion potential at or below a range of values of about -230 to -300 mV based on the standard hydrogen electrode (SHE) scale. IGSCC proceeds at an accelerated rate in systems in which the ECP is above the critical potential, and at a substantially lower or zero rate in systems in which the ECP is below the critical potential. Water containing oxidizing species such as oxygen increases the ECP of metals exposed to the water above the critical potential, whereas water with little or no oxidizing species present results in an ECP below the critical potential.
Corrosion potentials of stainless steels in contact with reactor water containing oxidizing species can be reduced below the critical potential by injection of hydrogen into the water so that the dissolved concentration is about 50 to 100 ppb or greater. For adequate feedwater hydrogen addition rates, conditions necessary to inhibit IGSCC can be established in certain locations of the reactor. Different locations in the reactor system require different levels of hydrogen addition. Much higher hydrogen injection levels are necessary to reduce the ECP within the high radiation flux of the reactor core, or when oxidizing cationic impurities, e.g., cupric ion, are present.
It has been shown that IGSCC of Type 304 stainless steel used in BWRs can be mitigated by reducing the ECP of the stainless steel to values below -0.230 V(SHE). An effective method of achieving this objective is to use HWC. However, high hydrogen additions, e.g., of about 200 ppb or greater, that may be required to reduce the ECP below the critical potential, can result in a higher radiation level in the steam-driven turbine section from incorporation of the short-lived N-16 species in the steam. For most BWRs, the amount of hydrogen addition required to provide mitigation of IGSCC of pressure vessel internal components results in an increase in the main steam line radiation monitor by a factor of five. This increase in main steam line radiation can cause high, even unacceptable, environmental dose rates that can require expensive investments in shielding and radiation exposure control. Thus, recent investigations have focused on using minimum levels of hydrogen to achieve the benefits of HWC with minimum increase in the main steam radiation dose rates.
An effective approach to achieve this goal is to either coat or alloy the stainless steel surface with palladium or any other platinum group metal. The presence of palladium on the stainless steel surface reduces the hydrogen demand to reach the required IGSCC critical potential of -0.230 V(SHE). The techniques used to date for palladium coating include electroplating, electroless plating, plasma deposition and related high-vacuum techniques. Palladium alloying has been carried out using standard alloy preparation techniques. Both of these approaches are ex-situ techniques in that they cannot be practiced while the reactor is in operation.
U.S. Pat. No. 5,135,709 to Andresen et al. discloses a method for lowering the ECP on components formed from carbon steel, alloy steel, stainless steel, nickel-based alloys or cobalt-based alloys which are exposed to high-temperature water by forming the component to have a catalytic layer of a platinum group metal. As used therein, the term "catalytic layer" means a coating on a substrate, or a solute in an alloy formed into the substrate, the coating or solute being sufficient to catalyze the recombination of oxidizing and reducing species at the surface of the substrate; and the term "platinum group metal" means metals from the group consisting of platinum, palladium, osmium, ruthenium, iridium, rhodium, and mixtures thereof.
In nuclear reactors, ECP is further increased by higher levels of oxidizing species, e.g., up to 200 ppb or greater of oxygen in the water, from the radiolytic decomposition of water in the core of the nuclear reactor. The method disclosed in U.S. Pat. No. 5,135,709 further comprises providing a reducing species in the high-temperature water that can combine with the oxidizing species. In accordance with this known method, high concentrations of hydrogen, i.e., about 100 ppb or more, must be added to provide adequate protection to materials out of the reactor core, and still higher concentrations are needed to afford protection to materials in the reactor core. It is also known that platinum or palladium can be added to increase the ECP of stainless steel exposed to deaerated acidic aqueous solutions, thereby forming a passive oxide layer on the stainless steel and reducing further corrosion.
The formation of a catalytic layer of a platinum group metal on an alloy from the aforementioned group catalyzes the recombination of reducing species, such as hydrogen, with oxidizing species, such as oxygen or hydrogen peroxide, that are present in the water of a BWR. Such catalytic action at the surface of the alloy can lower the ECP of the alloy below the critical potential where IGSCC is minimized. As a result, the efficacy of hydrogen additions to high-temperature water in lowering the ECP of components made from the alloy and exposed to the injected water is increased manyfold. Furthermore, it is possible to provide catalytic activity at metal alloy surfaces if the metal substrate of such surfaces contains a catalytic layer of a platinum group metal. Relatively small amounts of the platinum group metal are sufficient to provide the catalytic layer and catalytic activity at the surface of the metal substrate. For example, U.S. Pat. No. 5,135,709 teaches that a solute in an alloy of at least about 0.01 wt %, preferably at least 0.1 wt % provides a catalytic layer sufficient to lower the ECP of the alloy below the critical potential. The solute of a platinum group metal can be present up to an amount that does not substantially impair the metallurgical properties, including strength, ductility, and toughness of the alloy. The solute can be provided by methods known in the art, for example by addition to a melt of the alloy or by surface alloying. In addition, a coating of the platinum group metal, or a coating of an alloy comprised of a solute of the platinum group metal as described above, provides a catalytic layer and catalytic activity at the surface of the metal. Suitable coatings can be deposited by methods well known in the art for depositing substantially continuous coatings on metal substrates, such as plasma spraying, flame spraying, chemical vapor deposition, physical vapor deposition processes such as sputtering, welding such as metal inert gas welding, electroless plating, and electrolytic plating.
Thus, lower amounts of reducing species such as hydrogen are effective to reduce the ECP of the metal components below the critical potential, because the efficiency of recombination of oxidizing and reducing species is increased manyfold by the catalytic layer. Reducing species that can combine with the oxidizing species in the high-temperature water are provided by conventional means known in the art. In particular, reducing species such as hydrogen, ammonia, or hydrazine are injected into the feedwater of the nuclear reactor.